It is now common practice to explore the oceans of the earth for deposits of oil, gas and other valuable minerals by seismic techniques in which an exploration vessel imparts an acoustic wave into the water, typically by use of a compressed air "gun." The acoustic wave travels downwardly into the sea bed and is reflected at the interfaces between layers of materials having varying acoustic impedances. The wave travels back upwardly where it is detected by microphone or "hydrophone" elements in a streamer towed by the vessel to yield information regarding characteristics of the underwater material and structures.
A towed streamer comprises a plurality of pressure-sensitive hydrophone elements enclosed within a waterproof jacket and electrically coupled to recording equipment onboard the vessel. Due to its extreme length (on the order of miles), the streamer is often divided into a number of separate sections or "modules" that can be decoupled from one another and that are individually waterproof. Individual streamers can be towed in parallel through the use of paravanes to create a two dimensional array of hydrophone elements. Data buses running through each of the modules in the streamer carry the signals from the hydrophone elements to the recording equipment (so-called "acoustic data").
In addition to acoustic data, it is also important to collect and transmit data concerning operational status of the array to the vessel (so-called "nonacoustic data"). Nonacoustic data comprises physical characteristics of interest regarding the operation of each module, including whether water has invaded a module in the streamer, module temperature, module depth and power supply voltage.
Each hydrophone element within the streamer is designed to convert the mechanical energy present in pressure variations surrounding the hydrophone element into electrical signals. Most typically, this is done by constructing the hydrophone of a piezoelectric material, such as lead zirconate titanate ("PZT") and a means by which to amplify pressure variations to obtain the strongest possible signal (often by one or more diaphragms acting as tympanic collectors). The hydrophone elements are typically provided with leads or contacts to which to join electrical conductors, the electrical conductors carrying signals from the hydrophone elements to the recording equipment.
During operation, hydrophones encounter acoustic noise produced by a wide variety of sources emanating from the surrounding ocean, such as surface ocean waves striking the streamer or its towing vessel, propeller noise or even volcanos. Thermal variations within the streamer itself of even fractions of a degree can bring about thermal stress in the PZT material of the hydrophone, causing additional noise. The noise these sources produce lies mostly in the range of below 10 Hz, increasing dramatically as the frequency approaches 0 Hz. The valid acoustic signals reflected back from the ocean floor tend to lie in a range from a few Hz to several hundred Hz.
It is desirable to remove very low frequencies because they use available dynamic range yet contribute very little information. This frees the buses of the burden of carrying data pertaining to the frequencies below 10 Hz, allowing the available dynamic range to be spent instead on a higher resolution of the data pertaining to the remaining higher frequencies.
In geophysical applications, it has been common to provide a third order high pass filter having a cutoff frequency in the range of 3 to 10 Hz to attenuate the low frequency noise. The hydrophone's internal capacitance and amplifier input resistance provided the first pole. A second-order filter comprising an operational amplifier having an impedance network associated therewith provided the remaining two poles and further filtered the hydrophone signals. The impedance network comprised a first impedance component coupling an output of the amplifier to a second impedance component, the second component coupled to an electrical ground. An inverted input of the amplifier was coupled to a point between the first and second components, resulting in a feedback loop. The hydrophone was coupled to a noninverting input on the amplifier. This prior art filtering arrangement will be discussed with reference to a drawing figure thereof in the detailed description to follow.
Unfortunately, this prior art scheme had two significant disadvantages. First, since the scheme relied upon the intrinsic capacitance of the hydrophone itself for the first pole, any variations in that capacitance from one hydrophone to another altered the filter transfer function and hence the characteristics of the filter. Second, impedance variations led to severe channel-to-channel phase mismatch.
Hydrophones are generally optimized for sensitivity and size, not for stable or consistent impedance characteristics. Therefore, such prior art schemes required hydrophones of known and stable impedances. Such hydrophones are commercially available, but are relatively expensive and may sacrifice other performance characteristics to optimize impedance consistency. In the case of multiple hydrophones coupled to a single second-order filter, it was advantageous to impedance-match the hydrophones as a group. This required even more expense, time and effort during manufacture.
Second, and given that the hydrophone/amplifier resistance pole is moved down in frequency and an electronic filter substituted to provide the proper frequency cutoff, the second component (a shunt to ground) was required to be a relatively large capacitor, on the order of 1 to 100 microfarads. Such capacitors have two problems. First, they are relatively large. In a 2 inch diameter streamer, space is at a premium. Therefore, if a large capacitor can be avoided, then the resulting space can be used otherwise. Second, and more critically, such relatively large capacitors tend to be inaccurate. Any inaccuracy in capacitance results in a variation of the filter transfer function, again altering filter characteristics, as in the case with variations in hydrophone impedance.
Thus, the prior art has failed to provide a hydrophone/high-pass filter circuit that has a stable cutoff frequency, accomplished without relying on hydrophone impedance and without employing large, relatively inaccurate shunt capacitors.